The present invention relates to aqueous melamine resin dispersions comprising as discontinuous phase a melamine resin stabilized by a combination of a cationic protective colloid and an anionic protective colloid.
The invention further relates to formulations comprising these dispersions, to decorative sheets coated with these formulations, to woodbase materials coated with these decorative sheets, and to processes for preparing or producing the melamine resin dispersions, decorative sheets and coated woodbase materials.
Melamine resin solutions comprising melamine and formaldehyde are general knowledge from, for example, Kunststoff Handbuch, 2nd Edition 1988, Volume 10, pp. 41 to 49.
From these solutions it is common, by impregnating and coating paper, to produce melamine resin sheets which are used inter alia for coating woodbase materials in the furniture industry.
The processing industry and the end users impose various requirements on the melamine resins and the products produced from them regarding the processing properties and end properties of the melamine resins and products produced from them.
According to the prior art, a sheet of a decorative paper is impregnated with a melamine resin condensate in a one- or two-stage process, dried and subsequently laminated under pressure and with heating onto a support plate. In the one-stage process, the resin filling the paper and the resin forming the sealing surface are identical. In the two-stage impregnating process, the decorative paper is first filled with 50-100% of solid resin (based on the paper weight) and, directly or after initial drying, further resin is applied to the top and/or bottom of the paper sheet by dipping, knife coating or brushing. This makes it possible to use different grades of impregnating resin in the two impregnating stages. The paper sheet is preferably filled using relatively inexpensive urea-formaldehyde impregnating resins or mixtures of urea-formaldehyde and melamine-formaldehyde impregnating resins. The top layer, which is critical to the properties of the product, consists preferably of pure melamine resin.
As far as the processability of the melamine resins is concerned, a particular desire is that the melamine resins are readily dilutable in water even after prolonged storage; in other words, on dilution with water they should not form any tacky coagulum. Good dilutability in water is important because this property is a precondition for easy cleaning of transportation vessels and processing machines. Further, it is desired that the melamine resins do not form a skin during the drying of the impregnated paper sheet. Premature filming of the resin surface, or formation of a skin, is disadvantageous since it hinders the subsequent drying process and reduces the rate of drying, whereas the processor is particularly interested in very rapid drying and thus high productivity.
As far as the service properties of the sheets produced with the melamine resins is concerned, these sheets are intended to possess a certain degree of elasticity so that they can also be used to coat structured and profiled surfaces of woodbase material parts without cracking in the pressed-on sheets. Furthermore, the elasticity should be sufficient to ensure that swelling and shrinkage in the woodbase material, as may occur, for example, with a change in the ambient climate, again do not result in surface cracking. In addition to cracking resistance of this kind, the pressed-on melamine resin sheets should in addition be insensitive to humidity and, in particular, to water vapor.
Moreover, the surface coatings are intended to impart an impression of color which is as brilliant as possible. This is often countered, however, by the fact that the resins used to impregnate the decorative papers gray on curing and tend to develop white efflorescence, and thus attenuate the color effect of color-printed or colored decorative papers. This phenomenon occurs particularly with black decorative papers, which then in many cases no longer have the desired xe2x80x9cblacknessxe2x80x9d.
A further requirement made by furniture producers is that the impregnated products produced with the melamine resins form high-gloss surfaces when pressed onto furniture parts.
The melamine resin solutions known from the prior art are generally already well able to meet this profile of requirements. However, as far as their water dilutability following storage is concerned, these resin solutions appear to be still in need of improvement. It is also regarded as disadvantageous that following application to the decorative paper which is to be coated the resin solutions form films prematurely, which may impair the drying process by forming bubbles or dust and/or may lead to reduced machine speeds and defects in the sheet surfaces.
Also known from the prior art are aqueous dispersions of melamine resins which are already in a fully or partly cured state.
A similar process for preparing benzoguanamine-melamine-formaldehyde particles is described in U.S. Pat No. 3,945,980. The amino resin precondensate there is diluted until its water compatibility limit is exceeded, with the addition of polyvinyl alcohol, and is cured by means of heat and acidity.
Moreover, the preparation of melamine resin particles is described in European Patents EP 0 415 273 and EP 0 363 752. The starting material used in this case is a methanol-etherified melamine resin which is crosslinked by means of acidity and heat in the presence of a sulfonic acid polymer at concentrations of about 7%. These melamine resins are in practice unsuitable for producing decorative sheets since in the course of the production of the sheets or of the coated woodbase materials they give off methanol, which is unacceptable from the industrial hygiene standpoint.
Processes similar to the above are described in DD 224 602, JP 11021355, SU 441 272, JP 62068811, DD 248803 and DE 3 628 244. The products in all cases are fully cured, unmeltable and insoluble thermoset powders with a wide variety of particle sizes, which are recommended for use as calibration material, pigment, rheology modifier, filler, flame retardant, and flatting agent. The space-time yields are unsatisfactory in every case and the particle sizes, owing to the use of the protective colloid systems described, are in some cases severely scattered.
The preparation of melamine resin particles is known, furthermore, from U.S. Pat. No. 3,428,607. The preparation is described of cured melamine resin particles by stirring a M/F precondensate into an aqueous solution of protective colloids such as carboxymethylcellulose, gelatine, agar-agar, starch or alginates at melamine resin concentrations of 0.01%-10% and carrying out reaction at a pH of 6-8 and at the boiling temperature of the solvent.
U.S. Pat. No. 5,344,704 discloses aqueous mixtures containing precured melamine resin particles and an additional binder (e.g., sodium alginate or microcrystalline cellulose). Decorative papers are impregnated or coated with this mixture, dried, impregnated with melamine resin solutions and then cured to produce sheets used for surface coating. Said mixtures are prepared by fully or partly curing a melamine resin and subsequently grinding it to an average particle size of about 50 xcexcm and dispersing it together with a protective colloid in water or a melamine resin solution. A particular disadvantage of this process is that the dispersions coagulate rapidly owing to the size of the resin particles. Furthermore, the grinding of the partly cured resin is technically complex.
WO 97/07152 describes a process for preparing aqueous dispersions containing fully or partly cured melamine resins. For this purpose an aqueous melamine resin solution is admixed with an aqueous suspension of a water-insoluble protective colloid, such as microcrystalline cellulose, the melamine resin precipitating as a result of exceeding its solubility limit, and forming a stable dispersion. This mixture is subsequently reacted further, if desired, so that the melamine resin attains the desired degree of cure. The melamine resin particles obtained in this way have an average size of from less than 1 xcexcm to 700 xcexcm. It is noted, furthermore, that soaps and customary water-soluble protective colloids are unsuited to the preparation of appropriate melamine resin particles since in the case of soaps, this leads to technical problems in connection with the preparation of the melamine resin dispersions and in the case of protective colloids leads to fine dispersions of nonuniform size. The melamine resin dispersions stabilized with water-insoluble protective colloids such as microcrystalline cellulose, on the other hand, have the disadvantage that they can be prepared only with low solids contents of less than 40%, since above a concentration of only 3% the microcrystalline cellulose used leads to the formation of a thixotropic gel. At concentrations of more than 40%, based on the sum of melamine and formaldehyde, microcrystalline cellulose can no longer be used to prepare a fine dispersion having particle sizes of less than 30 xcexcm. Moreover, microcrystalline cellulose is known to be a relatively expensive ingredient.
It is an object of the present invention to provide melamine resin dispersions which do not have the disadvantages of the prior art and which in particular permit the production of sheets having superior surface properties such as luster and coherency.
We have found that this object is achieved by the initially defined melamine resin dispersions, formulations, decorative sheets and woodbase materials, and by processes for preparing and producing them.
The melamine resin dispersions of the invention are generally prepared from
a) melamine
b) from 1.3 to 3.0 mol of formaldehyde per mole of melamine,
c) if desired, up to 0.5 mol of urea per mole of melamine, and
d) if desired, from 0.01 to 0.3 mole of another compound capable of reacting with formaldehyde in a polycondensation reaction, per mole of melamine.
The melamine (component a) is normally used in solid form.
The formaldehyde (component b) is used preferably in the form of an aqueous solution with a strength of from 30 to 50% by weight or in the form of paraformaldehyde.
The urea (component c) is employed likewise in solid form, in the form of an aqueous solution or in the form of a precondensate with the formaldehyde.
Suitable components (d) are primarily those used, if desired, together with formaldehyde in the preparation of amino resins (cf. Ullmanns Encyklopxc3xa4die der technischen Chemie, 4th Edition, Volume 7, pp. 403 to 422), i.e., for example, dicyandiamide and guanamines such as benzoguanamine and acetoguanamine. Bisguanamines such as adipo-, glutaro- or methylolglutarobisguanamine, and compounds containing two or more rings, e.g. fused aminotriazine rings, are likewise suitable.
Ingredients suitable for use as elasticizers (component e) are the following:
mono- or polyhydric alcohols, e.g., tert-butanol, ethylene glycol, diethylene glycol, triethylene glycol, polyethylene glycols, butanediols, pentanediols, hexanediols, trimethylolpropane, neopentyl glycol and sorbitol
amino alcohols, e.g., ethanolamine, diethanolamine and triethanolamine,
amides and lactams, e.g., formamide, methylformamide, dimethylformamide, urea, methyl ureas, cyclic ureas, thio urea, polyureas, C3 to C9 lactams
ethanolamides, e.g., formic acid ethanolamide, acetic acid ethanolamide, and trishydroxyethyl isocyanurate-hydroxyethylurea,
the abovementioned compounds in ethoxylated form, said compounds carrying on average preferably from 1 to 20 ethylene oxide units, including in particular ethoxylated caprolactam, ethoxylated oligo- or polycaprolactam having on average from 1 to 10 ethylene oxide units per caprolactam unit, and also ethoxylated melamine,
2-(5-hydroxy-3-oxapentylamine)-1,3,5-triazine, 2,4-di(5-hydroxy-3-oxapentylamine)-1,3,5-triazine, 2,4,6-tris(5-hydroxy-3-oxapentylamine)-1,3,5-triazine (THOM) or mixtures of these compounds (HOM), and
further, the elasticizers specified in EP-A-800 543.
Particularly suitable cationic protective colloids are cationic starch, quaternized polyvinylimidazole and/or its copolymers with, for example, vinylpyrrolidone; quaternized polyethyleneimines and polyvinylamines.
The anionic protective colloid is preferably a copolymer of acrylic acid, methyl acrylate and 2-acrylamido-2-methylpropanesulfonic acid. Further suitable compounds are the following: sodium polyacrylate, sodium polymaleate, the sodium salts of copolymers of acrylic and maleic acid, phenolsulfonic acid-formaldehyde condensates, naphthalenesulfonic acid-formaldehyde condensates, and anionic starches.
The anionic and the cationic protective colloid are normally used in an overall amount of from 0.02 to 6% by weight, preferably from 0.5 to 3% by weight, based on the overall amount of melamine and formaldehyde used to prepare the melamine resin. The weight ratio of anionic to cationic protective colloid is usually from 0.02:1 to 50:1, preferably from 0.1:1 to 10:1.
The melamine resin dispersions are generally prepared by the following procedure:
1. in step 1, first preparing an aqueous solution of a melamine resin from components a) to d) in the presence or absence of the anionic and of the cationic protective colloid and continuing reaction at a pH of from 7 to 10, preferably from 7.2 to 9 and at a temperature of from 50 to 140xc2x0 C., preferably from 90 to 120xc2x0 C., with particular preference from 60 to 100xc2x0 C., until cloudiness begins, t he anioni c and cationic protective colloids being added no later than at th e o nset of clouding;
and
2. further reacting the melamine resin solution prepared in accordance with step 1 at a pH of from 7 to 10, preferably from 7.2 to 9 and at a temperature of from 50 to 140xc2x0 C. until it has undergone transition to a dispersion.
The pH is generally established using Brxc3x6nsted alkalis or acids, or buffer substances such as diethylethanolamine, for example.
The reaction can in principle take place in accordance with the same methods and in the same apparatus as the preparation of the melamine resin solutions known in general.
The formation of a disperse phase can be monitored very simply by visual assessment (onset of clouding) or very much more precisely by means of on-line turbidity measurement, up to a defined turbidity value, by means of a fiber-optical diffuse-light probe.
The mixing of the reaction mixture formed in step 1 with the protective colloids is not critical; the use of special stirring elements exerting a high shear action is not required.
Since the melamine resin dissolved initially has undergone transition to a dispersion, the reaction is ended by cooling to normal temperature and a pH of from 9 to 10 is established.
The melamine resin present in dispersed form in the melamine resin dispersions generally has a glass transition temperature of from 0 to 80xc2x0 C., preferably from 20 to 60xc2x0 C. and an energy content of from 20 to 160 J/g (calculated on the basis of a 100% dispersion).
The glass transition temperature and the energy content can be determined in a simple manner by means of DSC measurements (DSC: differential scanning calorimetry).
When the desired energy content and glass transition temperature have been reached can be determined by means of a simple preliminary experiment in which a sample is taken every 5 minutes, approximately, in step 2 and is analyzed by DSC for its energy content and its glass transition temperature.
The elasticizers can in principle be added at any phase of the preparation or subsequent thereto. In the case of the alcohols and urea derivatives they are preferably added at the beginning of the reaction; in the case of the amides they are preferably added at a later point in time. The time of addition and the reaction regime determine the incorporation of the elasticizers into the resin matrix and influence the product properties via the resulting difference in distribution in the dispersion phase or in the continuous phase.
As far as the reasons for the turbidity and the action of the protective colloid are concerned, the following is presumed:
The formation of a disperse phase results from the increasing hydrophobicization of the amino resin component in the course of condensation. The increase in the hydrophobic nature of the amino resin component results from the formation of higher-order aggregates, without the formation of covalent bonds between methylol compounds, and/or the increase in the molecular weight as a result of the formation of covalent bonds.
The contribution made by the two mechanisms to hydrophobicization determines the properties of the dispersion. The two mechanisms can be differentiated by analyzing the products and the intermediates using quantitative 13C NMR spectroscopy and DSC. Quantitative 13C NMR spectroscopy can be used to determine the proportion of methylene bridges, methyl ether bridges, and methylol groups. The DSC method can be used to determine the glass transition temperature and the energy content of the dispersion and to draw conclusions regarding the degree of condensation. Particularly desirable is a high proportion of methylol groups ( greater than 40 mol % based on formaldehyde CH2 groups detected) and a glass transition temperature and energy content as defined. Lower energy contents result in dispersions having inadequate properties.
The aggregation of the resin phase in the course of its deposition is prevented by the protective colloids used. The cationic protective colloid occupies the interface formed and initially stabilizes the dispersion. The anionic protective colloid then in turn occupies the cationic interface, leading to charge reversal. In addition to the electrostatic stabilization which this achieves, excess anionic protective colloid contributes to stabilizing the dispersion by way of the mechanism of depletion stabilization. Without the addition of the anionic protective colloid, the dispersion thickens on storage and forms dilatant sediments. Without the cationic protective colloid, the dispersions obtained are coarse with a broad particle size distribution (0.5 xcexcm to 500 xcexcm).
The reaction mixture preferably contains water in amounts such that the solids content of the reaction mixture is from 40 to 70% by weight. Since this is also the preferred value for the melamine resin dispersion, it is then unnecessary to dilute it further by adding water or to concentrate it by means of distillation under reduced pressure.
The viscosity of the melamine resin dispersions prepared in this way is generally from 20 to 300 mPas (based on a dispersion having a solids content of 50%, measured at 20xc2x0 C.).
The particle sizes are on average from 0.05 xcexcm to 300 xcexcm, preferably from 0.2 xcexcm to 5 xcexcm.
The melamine resin dispersions are generally employed in the form of formulations comprising
the melamine resin dispersion of the invention
if desired, from 1 to 200% by weight, based on the overall amount of formaldehyde and melamine in the form of the melamine resin, of an uncured melamine resin and/or urea resin in dissolved form
if desired, a thickener or thixotropic agent.
Suitable melamine or urea resin solutions with which the melamine resin dispersions are used in the form of the formulations are commercially customary products as recommended for paper impregnation.
The formulations are suitable as impregnating materials, especially for producing melamine resin impregnated products. They can be used in a similar way to the normal commercial melamine resin-based products for this application sector.
Such products are prepared by impregnating papers, known as impregnation papers, e.g., decoration papers and core layer papers (sodium kraft papers), with the melamine resins. In this utility the melamine resins are employed in the form of an aqueous solution with a strength of from 40 to 70% by weight, normally with the addition of a curing agent.
Suitable curing agents are Brxc3x6nsted acids such as organic sulfonic and carboxylic acids and their anhydrides, e.g., maleic acid, maleic anhydride and formic acid, ammonium compounds, e.g., ammonium sulfate, ammonium sulfite, ammonium nitrate, ethanolamine hydrochloride, and dimethylethanolammonium sulfite, and also curing agent combinations such as morpholine/p-toluene sulfonic acid.
The curing agents can be added in amounts of from 0.1 to 2.5% by weight, based on the aqueous impregnating resin. The skilled worker is aware that the amount of curing agent can be adapted to the particular performance requirements, it being possible to make corresponding adjustments to the reactivity of the impregnating resin/curing agent mixtures by way, for example, of measurement of the turbidity times and gelling times.
Auxiliaries such as wetting agents may further be added to the impregnating liquors. Examples of suitable wetting agents are ethoxylated fatty alcohols or alkylphenol ethoxylates, which can be added in amounts of from 0.2 to 0.6% by weight, based on the resin solution.
The manner in which the impregnating liquors are processed further to melamine resin impregnated products, and in which the woodbase materials are coated with these impregnated products, is known to the skilled worker.
Possible processes for the further processing of the impregnating liquors to melamine resin impregnated products composed of a plurality of superposed papers, e.g., high-pressure laminates (HPL) and continuously produced laminates (CPL), are described, for example, in DE-A-41 39 961 and DE-A 42 40 982.
The formulations of the invention can be processed either by the one-stage or by the two-stage process. In the case of the one-stage process, the resin filling the paper is present in the continuous phase of the blend, with the sealing surface being formed by the resin of the disperse phase. In the case of the two-stage impregnating process, the decoration paper is first filled with 50-100% of solid resin (based on the paper weight) and, directly or after initial drying, further resin is applied to the top and/or bottom of the paper sheet by dipping, knife coating or brushing. The paper sheet is preferably filled using relatively inexpensive urea-formaldehyde impregnating resins or mixtures of urea-formaldehyde and melamine-formaldehyde impregnating resins. The top layer, which is critical to the properties of the product, consists preferably of pure melamine resin.
The films or sheets produced in this way are normally pressed self-adhesively onto woodbase substrates under pressure and at temperatures  greater than 120xc2x0 C. or else are glued on with the aid of adhesives.
The pressing of these films or sheets onto stock made of different materials such as wood, polymers, fiber composites or, in particular, woodbase materials, e.g., plywood, wood fiber board and, in particular, chipboard, gives said stock a surface which is crack-resistant, glossy and insensitive to water vapor.
One of the principal advantages of the resin dispersions of the invention is that the melamine resin is not primarily in the form of colloidal solutions and so does not have significant weaknesses of the prior art systems, such as poor storage stability, limited water dilutability, and premature filming in the course of drying.
Experimental Section
A. Preparation of the Melamine Resin Dispersions